As a recent electronic technology progresses, portable electronic equipment such as a cellular phone, a notebook-sized PC, audio/visual equipment, and mobile terminal equipment rapidly comes into proliferation. Such portable electronic equipment is conventionally driven with a secondary battery. New-type secondary batteries replace exiting models while achieving a downsizing and reduction in weight, and increasing in energy density. For example, the secondary cell has been developed as a sealed lead battery, an Ni—Cd battery, a nickel hydrogen battery, and a lithium ton battery. In any of the secondary batteries, battery active materials and a larger-capacity battery structure are being developed to increase the energy density, and efforts are made to realize a power supply of longer use time.
Although efforts to realize lower power consumption are being made in portable electronic equipment, as new functions thereof are continuously added to respond to customer's needs, it is expected that total power consumption of portable equipment is increasing from now on. Consequently, there is a trend toward a higher-density power supply, that is, a power supply of longer continuous use time.
Expectations for a power supply unit using a fuel cell to assure longer use time of a portable equipment for the user are rising. As the fuel cell for the portable equipment, a direct methanol fuel cell (hereinbelow, DFMC) is regarded as a promising fuel cell since it can generate power even at room temperature. Different from the existing secondary batteries, the aim of the DFMC is to enable continuous power generation by supplying a fuel with a cartridge or the like.
To reliably perform continuous power generation, the concentration of methanol has to be controlled.    Reference: Translated National Publication of Patent Application No. 2004-537150
When a power supply unit with a DMFC is continuously used, the fuel concentration in a fuel storing mechanism (hereinbelow, called fuel tank) capable of storing a necessary amount of fuel near a power generating part of the DMFC changes. When the methanol concentration in the fuel which is in contact lies out from a predetermined range, drop in an output of the power generating part in the DMFC becomes large.
An object of the present invention is to stabilize an output of a power supply unit by controlling a fuel concentration in a fuel tank so as not to be out of a predetermined range.